An internal combustion engine is provided with an exhaust gas sensor (an air-fuel ratio sensor, or an oxygen sensor) for detecting an air-fuel ratio or a rich/lean state of an exhaust gas in an exhaust pipe. A fuel injection quantity is feedback-controlled so that the air-fuel ratio of the exhaust gas agrees with a target air-fuel ratio on the basis of an output of the exhaust gas sensor. Generally, since the detection precision of the exhaust gas sensor is low until the temperature of the sensor element rises to activation temperature, the sensor element is heated by a heater provided in the exhaust gas sensor to promote activation of the exhaust gas sensor.
Exhaust gas of an internal combustion engine contains water vapor generated by combustion reaction between fuel and air. When the temperature of the exhaust pipe is low immediately after starting period of the internal combustion engine, the exhaust gas containing the water vapor is cooled down in the exhaust pipe. In some cases, the water vapor in the exhaust gas in the exhaust pipe is condensed and condensate water is generated. Consequently, there is a possibility that the condensate water generated in the exhaust pipe immediately after starting period of engine adheres to the sensor element in the exhaust gas sensor. When the sensor element is heated by the heater immediately after starting period of engine, the high-temperature sensor element heated by the heater may be broken due to a local cooling (heat distortion) caused by adhesion of the condensate water. This is referred to as an element breaking, hereinafter.
JP-2003-328821A shows a heater energization delay time (heater off time) is established according to a coolant temperature during starting period of engine. When the delay time has elapsed, it is determined that the exhaust pipe temperature has risen to a temperature at which no condensate water is generated in the exhaust pipe, so that an energization of the heater is started.
Further, to increase the precision of the heater energization delay time, as described in JP-2007-321561A, reference heater energization delay time is established according to the lowest temperature among coolant temperature, intake-air temperature, and ambient temperature during starting period of engine. Correction time is established according to a difference between the coolant temperature and the ambient temperature (or a difference between the coolant temperature and the intake-air temperature). A final heater energization delay time is established by correcting the reference heater energization delay time with the correction time. After the final heater energization delay time has passed, the energization of the heater is started.
As described in JP-2007-120390A, while regulating energization duty of a heater, electric current is applied to the heater so as to preheat a sensor element of an exhaust gas sensor at a temperature at which no element breaking due to water adhesion occurs until a predetermined preheat period is elapsed. After the preheat period is elapsed, the energization duty of the heater is increased to make the temperature of the sensor element rise to the activation temperature.
An exhaust heat quantity after engine starting period changes according to an engine condition after starting period. A time necessary for the temperature of the exhaust pipe to rise to a temperature at which no condense water is generated in the exhaust pipe (that is, heater energization delay time necessary to prevent the element breaking of the exhaust gas sensor) changes.
In JP-2003-328821A and JP-2007-321561A, the heater energization delay time is established according to only engine condition during starting period such as a coolant temperature without considering a change in the exhaust heat quantity according to an engine condition after starting period. Consequently, the heater energization delay time has to be established to rather long time in consideration of an allowance on a safety side. A start of energizing the heater delays, an activation of the exhaust gas sensor delays, a start of air-fuel-ratio feedback control delays, and exhaust emission deteriorates.
JP-2002-48749A shows that an exhaust gas heat quantity (or exhaust gas temperature) is calculated on the basis of an operation state of the internal combustion engine. The exhaust pipe temperature is estimated on the basis of the exhaust gas heat quantity (or the exhaust gas temperature) and a heat transfer model obtained by mathematically modeling a heat transfer between exhaust gas and the exhaust pipe and a heat transfer between the exhaust pipe and the outside air. When the exhaust pipe temperature rises to a temperature at which water vapor in the exhaust gas is not condensed in the exhaust pipe, the energization of the heater is started.
However, a computing process necessary to estimate exhaust pipe temperature using the heat transfer model is complicated. A computation load on the controller increases, and the high precision of estimation of the exhaust pipe temperature is necessary.
In JP-2007-120390A, the heater energization duty is established only according to an engine condition during starting period without respect to a change in the exhaust heat quantity due to a change in the engine condition during a preheat period. Consequently, the heater energization duty has to be established to a rather low energization duty on the safe side. Accordingly, a preheat temperature of the sensor element becomes low, a time to increase the temperature of the sensor element to activation temperature after the preheat period becomes long, a start of air-fuel-ratio feedback control delays, and an exhaust emission deteriorates.